In textile laboratories, especially of spinning mills, spot checks are carried out on random samples by way of quality control to determine certain textile parameters such as fluctuations in weight and other characteristic quantities which can be derived from these measurements. These tests are carried out by means of so called uniformity testers of the kind, for example, which are distributed world wide by Zellweger Uster AG under the registered trademark of USTER.
Typically a uniformity tester includes a measuring unit containing a guide device, a measuring instrument, a feed device and a draw-off device for the sample to be tested. The feed device is formed by a pair of transport rollers. In the measuring instrument, the strand is sensed while passing through one of a plurality of measuring gaps each of which constitute the air gap of a capacitor and the aforesaid magnitudes are measured capacitively.
To determine the fluctuations in weight of the material being tested, the material is pulled through the measuring comb by the transport rollers and is removed by suction by the draw off device after the test. The rate of feed is up to 400 meters per minute for test material consisting of staple fibres and up to 800 meters per minute for filament yarns. Under these conditions, perfect operation of the transport rollers is of major importance.
In the known uniformity testers, one of the two transport rollers is motor driven while the other is driven by friction with the driven roller or, in the case of thick test material, it is driven by way of this material. The roller which is not actively driven therefore always rotates more slowly than the driven roller. The difference in the circumferential speeds of the two rollers depends on the fineness of the material being tested. The operator therefore has to deal with undetectable variations in speed which lead to unreliable or false measurement results.
Another factor which influences the rate of feed of the test material is the extent to which the transport rollers, which are made of a hard rubber-like material, have undergone wear. In the known uniformity testers, a manually operated guide web is provided to prevent the wear on the rollers always taking place in the same position as this could lead to the formation of grooves. The position of this guide web should be adjusted by the operator before each test so that the material to be tested will always pass through the nip between the rollers at a different point.
Apart from the fact that this displacement of the guide web does not ensure that the transport rollers will wear uniformly, the displacement is not always carried out regularly by the operators.